Device for photographic monitoring of road intersections controlled by a traffic light

ABSTRACT

At an intersection controlled by a traffic light, a first sensor provides an electric signal when a vehicle enters the intersection against the red light. That electric signal causes a first photograph to be immediately taken of the intersection and a second photograph of the intersection to be taken after a time delay. If that vehicle illegally entering the intersection turns right, it activates a second sensor which sends a signal to cause another picture to be taken. A second vehicle illegally entering behind the first will not interrupt the sequence of pictures initiated by the first, but will result in another picture being taken after a predetermined time lapse.

United States Patent [1 1 Holzapfel DEVICE FOR PHOTOGRAPI-IIC MONITORING OF ROAD INTERSECTIONS CONTROLLED BY A TRAFFIC LIGHT [75] Inventor: Jiirgen Holzapfel, Neuss, Germany [73] Assignee: Robot F oto und Electronic GmbI-I &

Co. KG, Dusseldorf-Benrath, Germany [22] Filed: Jan. 24, 1974 [21] Appl. No.: 436,036

[30] Foreign Application Priority Data [58] Field of Search 346/107 UP, 40; 340/31 C; 354/76, 105, 106, 354

[56] References Cited UNITED STATES PATENTS 2,871,088 l/l959 Abell 346/107 UP X [4 1 Dec.3l, 1974 3,060,434 10/1962 Biedennann et al. 346/107 UP Primary Examiner-Joseph W. Hartary Attorney, Agent, or Firm--Darbo, Robertson & Vandenburgh [5 7] ABSTRACT At an intersection controlled by a traffic light, a first sensor provides an electric signal when a vehicle enters the intersection against the red light. That electric signal causes a first photograph to be immediately taken of the intersection and a second photograph of the intersection to be taken after a time delay. If that vehicle illegally entering the intersection turns right, it activates a second sensor which sends a signal to cause another picture to be taken. A second vehicle illegally entering behind the first will not interrupt the sequence of pictures initiated by the first, but will result in another picture being taken after a predetermined time lapse.

8 Claims, 3 Drawing Figures SHEET 20F 3 1 DEVICE FOR PHOTOGRAPIIIC MONITORING OF ROAD INTERSECTIONS CONTROLLED BY A TRAFFIC LIGHT BACKGROUND AND SUMMARY OF THE I INVENTION This invention relates to a device for photographic monitoring of road intersections at which there is a traffic light, in which device a photographic camera is triggered by a sensor responding to a vehicle entering the intersection during the stop period of the traffic light, and in which device there is also provision for triggering a second camera exposure.

There are prior art devices for photographic monitoring of road intersections controlled by a traffic light. in the prior art devices, a sensor is positioned at a point along the road near the traffic light. This sensor supplies a signal when a vehicle passes over it. If the signal appears during the stop interval of the traffic light, i.e. a vehicle has entered the road intersection during the stop interval contrary to traffic regulations, a photographic camera will be triggered. The field of view of this camera covers the road intersection. Thereby a photograph is made of the traffic violation and also of the license plate of the violating vehicle (German patent 683,658). I

At times it will be inevitable that a vehicle driving toward the road intersection shortly before the traffic lights change from go to stop cannot be stopped in front of the traffic lights but only shortly behind the traffic lights. Usually the driver cannot be blamed for this. With only a single exposure showing nothing but the vehicle across or behind the stop mark monitored by the sensor, the photograph of such a situation would not be different from a photograph of an occurrence in which the vehicle had actually gone on and entered the road intersection during the stop interval. Therefore, care must be taken to distinguish between these two cases. To this end, there is a prior art device which makes a second exposure a predetermineddelay time after the first exposure. The second exposure would show whether the vehicle continued on into the intersection and thus would provide clear evidence of whether a traffic violation occurred. In addition, a-

clockcan also be photographed with each exposure, whereby the time sequence of this event can be recorded conclusively (U.S. Pat. No. 2,871,088).

There is, however, a loophole in the prior art apparatus: If a fast vehicle enters the road intersection during the stop interval and immediately turns into the intersecting road, that vehicle may already have left the field of view of the monitoring camera when the second exposure is made. On the photograph taken with the second exposure, no vehicle will be seen. An evidence of a traffic violation, if any, can be derived therefrom only indirectly. For example, it can be argued that, when the traffic lights'changed to stop, the driver had been able to stop his vehicle only beside or shortly behind the traffic lights (first exposure) and that he then drove the vehicle backwards to a location in front of the traffic lights and out of the field of view of the camera at the lower edge of this field (second exposure).

It is an object of this invention to provide an apparatus of the type mentioned hereinbefore which will also conclusively record such vehicles which have entered the intersection during the stop interval and then have turned into the intersecting road.

According to the invention, a second sensor is positioned in the road, and an additional exposure is triggered if said second sensor responds within said delay time. Thus if the vehicle entering the road intersection during the stop interval turns into the intersecting road a second camera exposure will be triggered by the second sensor before the delay time has expired and before the vehicle has left the field of view of the camera. The second exposure will provide a conclusive record of the traffic violation.

In one embodiment of the invention, a first monostable multivibrator is connected to be triggered by the first sensor. This monostable multivibrator opens a first gate circuit in a control path for a trigger signal from the second sensor to the camera. The second exposure can be triggered by the first monostable multivibrator returning to its stable state after said predetermined delay time. Thus the second exposure will be made at the end of the delay time in any event and will occur also if the vehicle continues to go straight ahead and only touches the second sensor.

Advantageously, the first sensor is an induction loop which controls a detector circuit. The pulse from the detection circuit should have a duration of at least the time required by the whole length of the vehicle to pass over the sensor. A second monostable multivibrator which remains in its metastable state for a shorter interval than the duration of said pulse from the detection circuit is connected to be triggered by the front edge of the detection circuit pulse in order to reduce the pulse width. The output pulses of the second monostable multivibrator trigger said first monostable multivibrator directly or through further intermediate circuits.

Thus, an initial pulse is produced by the detector circuit. This pulse continues while the entire length of the vehicle passes over the induction loop. This makes sure that one vehicle will not produce a plurality of sensor pulses, for example one for each axle. I This initial pulse is, however, too long for the actua tion of the processing circuit. Therefore a second pulse of shorter duration is derived therefrom by the second monostable multivibrator. This shorter pulse can then be used in a manner whereby the output of the second monostable multivibrator triggers a third monostable multivibrator to produce a pulse signal triggering the first camera exposure. Also, the rear edge of the latter pulse signal is used to trigger said first monostable multivibrator.

According to a further modification of the invention, the signal triggering the second exposure is produced by a fourth monostable multivibrator connected to be triggered by the first monostable multivibrator returning to its stable state. A fifth monostable multivibrator is connected to the output of the fourth monostable multivibrator to be triggered by the rear edge of said exposure triggering signal, remains in its metastable state for an interval substantially equal to the film feed time, and blocks a gate circuit in said control path of said exposure triggering signal; Thus if a second vehicle passes over the first sensor shortly after a first vehicle has entered the road intersection, the second vehicle cannot immediately trigger an exposure. The exposure triggering pulse produced by the second vehicle is stored and triggers an exposure after a predeter- DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic plan view of a road intersection and the arrangement of traffic lights, sensors and camera;

FIG. 2 is a schematic diagram of the circuit used in an embodiment of the invention; and

FIG. 3 is a more detailed wiring diagram.

DESCRIPTION OF SPECIFIC EMBODIMENT The following disclosure is offered for public dissemination in return for the grant of a patent. Although it is detailed to ensure adequacy and aid understanding, this is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how others may later disguise it by variations in form or additions or further improvements.

FIG. 1 shows a road intersection formed by two roads l and 12 intersecting each other at right angles. The road intersection is controlled by a traffic light installation, only one of which is shown. This is light 14 which controls the lane of movement of vehicles from left to right in road 10. The present description only describes the apparatus employed in conjunction with this lane, but it will be'understood that corresponding apparatus is employed in conjunction with the remaining lanes. Pedestrian walks 16 have been marked on the road surface at the road intersection. There is also a stop mark 18 on the road surface in the lane of travel controlled by the respective traffic lights. The vehicles should stop in front of (i.e. immediately before reaching) the stop I mark 18 during the stop interval of the traffic lights.

The road intersection is automatically monitored by aphotographic camera'20, in order to make a conclusive photographic record of all vehicles which enter or cross the roadintersection during the stop interval. To

this end, there is a sensor in the form of an induction loop I, between the stop mark 18 and the pedestrian walk 16. The induction loop I, and the traffic lights 14 are connected to a control unit 23 which triggers the camera 20.

The technique of producing an exposure triggering pulse when a vehicle passes over an induction loop disposed on or. below the road surface is well known and,

therefore, will not'be described in detail;

The induction loop I, is connected to a detector circuit D, which produces a signal when a vehicle passes over the induction loop (FIG. 2). During the duration of the stop interval of the traffic lights a signal is applied to a control input 26 of a NAND-gate N Thus when a signal from detector circuit D, is received by NAND-gate N during the stop interval that signal triggers a monostable multivibrator MF, through an inverter N ,5. The monostable multivibrator MF, produces a trigger signal to be applied to the camera through a control path 28.

By returning into its stable state, the monostable multivibrator MF, triggers a monostable multivibrator MF, which changes to its metastable state for a predetermined delay time. When the monostable multivibrator MF, returns to its stable state it triggers a monostable multivibrator MP through a NAND-gate N -andan inverter N Monostable multivibrator MP also applies a triggering signal to the camera through a control path 30.

Thus a first exposure will be triggered by monostable multivibrator MF,, when a vehicle passes the induction loop I, during the stop interval of the traffic lights, and thereafter a second exposure will be triggered by the monostable multivibrator MF, after a delay time deter-- mined by the monostable multivibrator MP A'second sensor in the form of induction loop I, is located on the right-turn lane of road 12. Induction loop I, is connected to a detector circuit D which produces a signal when a vehicle passes over it. This signal is inverted by the inverter N and is applied to a second input of the NAND-gate N Normally (i.e. with no signal from sensor I the output of the detector circuit D, is in the state 0., Inverted this is a signal L'at the input of the NAND-gat'e N When the monostable multivibrator MF; is in its metastable state, it produces an L-signal at the other input. of the NAND-gate N The output of gate N2 is then 0, which is inverted to I. by inverter N When sensor I produces a signal due to a vehicle passing thereover, detector circuit D produces a signal L. This signal isinverted to O by inverter N Thus the output of the NA-N-D-gate N becomes L, which is inverted to O by inverter N Thus there will be a declining edge of the signal by which the monostable multivibrator MF is triggered the same way as if the monostable multivibrator MF,

returns to its stable state so that an additional exposure is triggered through control path'30.

Thus if a vehicle crosses sensor 1,, enters the road intersection during the stop interval and turns to the right 7 into road 12, it could have moved laterally out of the field of view of the camera at the moment of the normal second exposure. In order to get a photographic recording of such a vehicle and obtain unambiguous evidence of its violation of the traffic regulations, an additional exposure is triggered by sensor 1, to produce a photograph of the vehicle turning away.

The triggering of the exposure after the delay timehas expired is effected independently of the additional for each axle. The signal thus obtained is, however, too

long to control the subsequent circuit. It must, therefore, be shortened. To this end, the signal is applied by detector circuit D, to one input of NAND-gate N through leads 32, 34, and to one input of another NAND-gate N,, through lead 32 and lead 36. The function of NAND-gate N,, is explained below.

At the same time, a monostable multivibrator MF, comprising two transistors T T, is triggered by the output signal of the detector circuit D applied to the base of transistor T The monostable multivibrator is triggered by the front edge of the output signal from detector circuit D,. It applies a signal through leads 38, 40 and 42 to one input, respectively, of each of the NAND-gates N and N Multivibrator MP produces an output pulse to lead 38 which is relatively short as compared to the duration of the input pulse from detector circuit D,.

A signal is applied to an input E when the stop interval of the traffic light 14 commences. This signal is applied to the base of a transistor T which operates as an inverter. Transistor T controls a transistor T operating as an electronic switch. When the switch is opened, i.e. the transistor T is nonconducting, a capacitor C is charged through an adjustable resistor P,. The voltage across capacitor C controls a Schmitt trigger comprising transistors T T and T Thereby a predetermined time interval can be selected, during which a vehicle passing over the induction loop I will not trigger an exposure.

The output signal of the Schmitt trigger is applied through an inverter N the control line 26 and lines 44 and 46 to one input of each of the NAND-gates N and N The output of the NAND-gate N is applied through the inverter N to the input of the monostable multivibrator MP The monostable multivibrator MF supplies a triggering signal for triggering the first exposure of the camera This triggering signal applied to line 28 triggers a monostable multivibrator MP through NAND-gates N and N (the function of which will be described below) and through lead 48. Monostable multivibrator MP produces a signal through lead 50 to an amplifier (not shown) which energize the triggering solenoid of the camera 20.

When the monostable multivibrator MF returns into its stable state, a monostable multivibrator MP is triggered through lead 52. The monostable multivibrator MP remains in its metastable state during an interval equal to the desired delay time between the first and the second exposure. The monostable multivibrator MP is triggered by the rear edge of the output signal 48 by the rear edge of the applied signal, thus by the transition from L to O.

This output of the monostable multivibrator MF, (which is in the state 0 when the monostable multivibrator is in its metastable state) and the respective outputs of the monostable multivibrators MF MB and MP are connected to an NAND-gate N through leads 58, 60, 62 and 64, respectively. Thus the output of NAND-gate N is L, if anyone of the monostable multivibrators MP MP MP or MF, is in its metastable, i.e. triggered, state. This output L is made on O by an inverter N and applied through line 66 to a further input of the NAND-gate N Therefore the NAND-gate N is rendered non-conductive during the passage of the signal through the monostable multivibrators MF,, MP MP and MP In addition, the output L of gate N is directly applied through line 66 to a further input of the NAND-gate N This results in the gate circuit comprising NAND-gate N 14 and the inverter N being rendered conductive if all leads are on positive potential.

Thus if a second vehicle passes over the induction loop 1 after a first vehicle passes overthe induction loop I and causes exposure to be triggered through MF, and MP and before the second exposure has actually been made and the film has been advanced, the pulse generated by the second vehicle is not supplied to the chain of monostable multivibrators MP to MF, because gate N is blocked. Instead, thissecond signal is applied to a lead 70 through the gate circuit N N A monostable multivibrator MP is triggered through lead 70. The monostable multivibrator MP remains in from the monostable multivibrator MP through NAND-gate 23 and inverter 24. Monostable multivibrator MP supplies a triggering signal to the monostable multivibrator MF through line 30, NAND-gates N and N and lead 48. Monostable multivibrator MF, supplies a pulse to the triggering solenoid of the camera through lead 50.

When the monostable multivibrator MP returns to its stable state, a monostable multivibrator MP, is triggered through lead 54. Monostable multivibrator MP, is connected to remain in its unstable state during an interval substantially equal to the film feed time. Multivibrator MF, is to make sure'that no further pulse can be supplied to the triggering solenoid of the camera while the film is being advanced. To this end, the monostable multivibrator MF, renders NAND-gate N nonconductive to signals from MP MP or MP and thereby prevents the triggering of the monostable multivibrator MP The output of the monostable multivibrator MP, is in the state 0, when the monostable multivibrator MP, is in its metastable state, lead 56 extending from this output. Thus the output of NAND- gate N is L, regardless of which signal is applied to the other input of the NAND-gate N The monostable multivibrator MP is, however, triggered through lead its metastable state for a hold time of, for example, 1.3 seconds. After this hold time has expired, the monostable multivibrator MF is triggered through lead 72 by the monostable multivibrator MP returning into its stable state. The monostable multivibrator MF generates a triggering signal on a lead 74, which also triggers the monostable multivibrator MP through gates N and N and lead 48. This results in a pulse being applied to the triggering solenoid of the camera through lead 50.

Signal 0 appears on lead 74 when the monostable multivibrator MF is in its metastable state. In the stationary state, signal L is applied to lead 74. In the stationary state, there is also L-signal on lead 28 from monostable multivibratorMF, and on lead 30 from monostable multivibrator MF Thus 0 appears at the output of NAND-gate N and consequently L appears at the output of NAND-gate N If one of the monostable multivibrators MP MF, or MF, is triggered and changes over into its metastable state, the signal on the respective lead 74, 28 or 30 becomes 0. Thus the output of NAND-gate N becomes L and, provided the output of the NAND-gate N supplies the signal L, the signal from the output of NAND-gate N i.e. on lead 48, changes over to O. Thereby the monostable multivibrator MF will be triggered.

The output 76 of the monostable multivibrator MP is applied to a monostable multivibrator MP, to trigger the monostable multivibrator MP when the monostable multivibrator MP returns into its stable state. The monostable multivibrator MP, has the same function as the monostable multivibrator MP It is to prevent triggering of the camera during the film feed operation. Thus the monostable multivibrator MF, remains in its metastable state during a hold time substantially equal is in the state 0, when the monostable multivibrator MP is in its metastable-state, is connected to a NAND- gate N through lead 78. Lead 56 from the monostable multivibrator ME, also is connected to an input of gate Nzz- The output from NAN D-gate N is 0, when neither of the monostable multivibrators MF, and MP, is

an L-signalby inverter N and applied to the input of 8 V (monostable multivibrator MP being triggered), the pulse thereby produced will cause an additional exposure to be triggered, provided noneof the interlocking means previously describedbecomes operative.

Thus the device of the invention makes it possible to conclusively record traffic violations at road intersections controlled by traffic lights even under unfavorthe NAND-gate N llf one of the monostable-multivibrators MP or MP is triggered, the NAND-gate N will be rendered non-conductive. The outputs from the monostablemultivibrators MF MF and MB, which supply the signal 0, when the respective monostable multivibrator is in its metastable state, are applied respectively to the three inputs of a NAND-gate N The output of this NAND-gate becomes L, when one of the three monostable multivibrators MF MF or MP is triggered. This output L is then converted into an O-signal bythe inverter N this O-signal being applied to the input leads 82, 84 of the NAND-gates N and N respectively and rendering these NAND- gates non-conductive, In this case, all further input signal triggers a monostable multivibrator MF comprising transistors T, and T Thereby a short pulse, as

compared to the input signal, is produced as has been described in connection with the. monostable multivibrator-MP The output from the monostable'multivifi brator MF is inverted by. an inverter N and is applied toone input of each of the NAND-gates N and N The output from the monostable multivibrator MP is applied to the other input of NAND gate N The output from the monostable multivibrator MP is applied to the other input of the NAND-gate N Normally the output from the monostable multivibrator MF will be 0. It is inverted to L by inverter N In the stationary state, the outputs from the monostable multivibrators MP and MP are in the state 0.. Thus there will be a signal L-at the outputs of NAND-gates N 'and N and a signal 0 at the outputs of the NAND-gates N24 and N29,. Q 1

If the multivibrator multivibrators MP is triggered, the second input of N AND-gate N also becomes L,

whereby the output from the inverter N becomes L. Thus the monostable multivibrator MP which is triggered by the rear edge of its input signal, will be triggered, when' either the monostable multivibrator MP returns into its stable state and brings one input of NAND-gate N into the state 0, or when an output pulse from the monostable multivibrator MF appears and through the inverter N brings the other input of the NAND-gate N into the state 0. In. both cases a triggering signal is applied to line 30 by the monostable multivibrator.

The circuit comprising the monostable multivibrator MP NAND-gate N inverter N and monostable multivibrator MP operates in a corresponding manner.

Thus, if a vehicle passes over the sensor comprising induction loop 1 during the delay time between the first and thesecond exposure (monostable multivibrator MF being triggered) or during the delay time by I able conditions such as with a plurality of vehicles entering the road intersection in rapid succession and/or with vehicles turning off.

- I claim: u

1. An improvement for an apparatus for photographically monitoring an intersection of'two roads, at which there is a traffic light, by making photographs of vehicles entering the intersection against the stop signal of the traffic light, said apparatus including a first sensor positioned along one of the roads to produce a signal when a vehicle passes the first sensor, afirst device to produce a first camera-actuating pulse in response to said signal for taking an initial photograph,;and a second device to produce a second-camera-actuating pulse a predetermined delay time afterv said initial photo graph for taking a second, photograph, said improvement comprising: g

a second sensor positioned along the other of the two roads to produce a signal when a vehicle passes the second sensor; and I means connected to saidsecond sensor and to said apparatus for producing a camera-actuating pulse when a signal from the second sensor occurs within said given period of time after the signal from the first sensor.

2. An apparatus and improvement as set forth in.

brator triggered by the signal from the first sensor and connected to said first gate means to open the gate means for the passage of said second pulse. 3. An apparatus and improvement as set forth in claim 2, wherein said first multivibrator changes from its stable to its metastable state by said triggering, remainsin its metastable state for said predetermined period of time and then returns to its stable state; and said means'defining said control path being connected to said firstmultivibrator to produce said second camera-actuating signal upon said multivibrator returning to its stable state. 4. An apparatus and improvement as set forth in claim 2,

wherein said first sensor comprises an induction loop; including a detection circuit connected to thefirst sensor for producing a detection pulse in response to the signal from the first sensor which detection pulse has a duration of at least the time required by the vehicle to pass the first sensor; and

wherein said first device comprises a second monostable multivibrator which remains in its metastable'state for a shorter period of time than the duration of said detection pulse, means connecting said second multivibrator to said detection circuit to trigger the second multivibrator from its stable state to its metastable state by the initiation of said detection pulse, and means connecting said second multivibrator to the first multivibrator to normally triggerthe latter when the second multivibrator returns to its stable state.

5. An apparatus and improvement as 'set forth in claim 4, wherein said first device includes a third monostable multivibrator having an input andan output, said second multivibrator being connected to said input whereupon the third multivibrator produces a pulse signal at its output in response to a signal at its input, means connecting said output of said third multivibrator t the first multivibrator to trigger the latter by the rear edge of said output pulse signal, and means connected to said output of said third multivibrator to use said output pulse signal to form said first cameraactuating pulse.

6. An apparatus and improvement as set forth in claim 5,

wherein said means defining said control paths for said camera-actuating pulses includes second gate means; and

wherein said second device includes a fourth monostable multivibrator having an input and an output, means connecting the input of the fourth multivibrator to the first multivibrator to trigger the former by the latter returning to its stable state to thereby produce a signal at the output of the fourth multivibrator for forming said second cameraactuating pulse, a fifth monostable multivibrator having a metastable state duration substantially equal to the film feed time, means connecting said fifth multivibrator to the fourth multivibrator to be triggered by the rear edge of said output signal of the fourth multivibrator, and means connecting the second gate means to said fifth multivibrator to close said second gate means when the fifth multivibrator is in its metastable state to thereby prevent a camera-actuating pulse from being formed during the film feed time following said production of said signal at the output of the fourth multivibrator.

7. An apparatus and improvement as set forth in claim 6 including:

means connected to the first, third, fourth and fifth multivibrators and defining a third gate means connected between the second and third multivibrators for blocking signal passage between the latter during signal passage through said first, third, fourth and fifth multivibrators;

fourth gate means connected to said first device which is open and permits a signal to pass therethrough to produce a detection pulse when said third gate means is blocking,

a pulse delaying sixth monostable multivibrator connectedto said third gate means to delay said passed signal; and

a seventh monostable multivibrator connected to said sixth multivibrator for producing a cameraactuating pulse in response to said delayed passed signal.

8. An apparatus and improvement as set forth in claim-1, wherein said second sensor is positioned on said second road and to the right of said first road from said first sensor. 

1. An improvement for an apparatus for photographically monitoring an intersection of two roads, at which there is a traffic light, by making photographs of vehicles entering the intersection against the stop signal of the traffic light, said apparatus including a first sensor positioned along one of the roads to produce a signal when a vehicle passes the first sensor, a first device to produce a first camera-actuating pulse in response to said signal for taking an initial photograph, and a second device to produce a second camera-actuating pulse a predetermined delay time after said initial photograph for taking a second photograph, said improvement comprising: a second sensor positioned along the other of the two roads to produce a signal when a vehicle passes the second sensor; and means connected to said second sensor and to said apparatus for producing a camera-actuating pulse when a signal from the second sensor occurs within said given period of time after the signal from the first sensor.
 2. An apparatus and improvement as set forth in claim 1, wherein said second device comprises means defining a control path for said second camera-actuating pulse and including a first gate means, and said first device comprises a first monostable multivibrator triggered by the signal from the first sensor and connected to said first gate means to open the gate means for the passage of said second pulse.
 3. An apparatus and improvement as set forth in claim 2, wherein said first multivibrator changes from its stable to its metastable state by said triggering, remains in its metastable state for said predetermined period of time and then returns to its stable state; and said means defining said control path being connected to said first multivibrator to produce said second camera-actuating signal upon said multivibrator returning to its stable state.
 4. An apparatus and improvement as set forth in claim 2, wherein said first sensor comprises an induction loop; including a detection circuit connected to the first sensor for producing a detection pulse in response to the signal from the first sensor which detection pulse has a duration of at least the time required by the vehicle to pass the first sensor; and wherein said first device comprises a second monostable multivibrator which remains in its metastable state for a shorter period of time than the duration of said detection pulse, means connecting said second multivibrator to said detection circuit to trigger the second multivibrator from its stable state to its metastable state by the initiation of said detection pulse, and means connecting said second multivibrator to the first multivibrator to normally trigger the latter when the second multivibrator returns to its stable state.
 5. An apparatus and improvement as set forth in claim 4, wherein said first device includes a third monostable multivibrator having an input and an output, said second multivibrator being connected to said input whereupon the third multivibrator produces a pulse signal at its output in response to a signal at its input, means connecting said output of said third multivibrator to the first multivibrator to trigger the latter by the rear edge of said output pulse signal, and means connected to said output of said third multivibrator to use said output pulse signal to form said first camera-actuating pulse.
 6. An apparatus and improvement as set forth in claim 5, wherein said means defining said control paths for said camera-actuating pulses includes second gate means; and wherein said second device includes a fourth monostable multivibrator having an input and an output, means connecting the input of the fourth multivibrAtor to the first multivibrator to trigger the former by the latter returning to its stable state to thereby produce a signal at the output of the fourth multivibrator for forming said second camera-actuating pulse, a fifth monostable multivibrator having a metastable state duration substantially equal to the film feed time, means connecting said fifth multivibrator to the fourth multivibrator to be triggered by the rear edge of said output signal of the fourth multivibrator, and means connecting the second gate means to said fifth multivibrator to close said second gate means when the fifth multivibrator is in its metastable state to thereby prevent a camera-actuating pulse from being formed during the film feed time following said production of said signal at the output of the fourth multivibrator.
 7. An apparatus and improvement as set forth in claim 6 including: means connected to the first, third, fourth and fifth multivibrators and defining a third gate means connected between the second and third multivibrators for blocking signal passage between the latter during signal passage through said first, third, fourth and fifth multivibrators; fourth gate means connected to said first device which is open and permits a signal to pass therethrough to produce a detection pulse when said third gate means is blocking, a pulse delaying sixth monostable multivibrator connected to said third gate means to delay said passed signal; and a seventh monostable multivibrator connected to said sixth multivibrator for producing a camera-actuating pulse in response to said delayed passed signal.
 8. An apparatus and improvement as set forth in claim 1, wherein said second sensor is positioned on said second road and to the right of said first road from said first sensor. 